1. Field of Invention
The present invention related to a semiconductor device having an insulative film and to a process for producing the same. The present invention relates more specifically to employment of a novel insulative film for a gate insulation film for MISFETs, for the dielectric element of a capacitor, for a passivation film for semiconductor devices, and for a mask for selectively forming circuit elements of semiconductor devices, and as well as to a novel process for forming the insulative film in the production of semiconductor devices.
2. Description of Prior Art
A. General
The material used as an insulative film in a silicon semiconductor device in the prior art includes silicon dioxide (SiO.sub.2), silicon nitride (Si.sub.3 N.sub.4) and silicon oxynitride (SiO.sub.x N.sub.y). The silicon dioxide is formed by a thermal oxidation of the silicon substrate or CVD process and is inherently not dense. The silicon nitride is formed by CVD or a direct nitridation of silicon. The silicon oxynitride is formed by a CVD process using NH.sub.3, O.sub.2 and SiH.sub.4 gases.
The insulative film previously used in a semiconductor device having a silicon substrate also includes a double film, wherein alumina (Al.sub.2 O.sub.3), silicon nitride or a mixture of Al.sub.2 O.sub.3 and Si.sub.3 N.sub.4 is applied or laminated on the silicon dioxide film. The known insulative single or double film is, however, unstable during the production or operation of the semiconductor device, and does not have a dense structure. Furthermore, it is inevitable that an interface between the insulation film and the silicon substrate is caused by the CVD process.
In the known CVD process of silicon oxynitride, monosilane (SiH.sub.4), ammonia (NH.sub.3) and oxygen (O.sub.2) are brought into reaction with each other at a temperature of, for example, 700.degree. C. The molar ratio of ammonia with respect to oxygen may be varied or increased gradually during the CVD period. However, the silicon oxynitride produced by this CVD process disadvantageously possesses structural defects. The inventors of the present invention believe that the structural defects are caused by the fact that intermediate products of the CVD reaction are incorporated into the insulation film.
A further disadvantage of the CVD process mentioned above is that it is difficult to precisely adjust the thickness of the silicon oxynitride film, and to form a film having a thickness of from 100 angstroms or less.
B. Surface Passivation
It is well known in the field of semiconductor devices to use silicon dioxide (SiO.sub.2) as an insulative film on a semiconductor silicon substrate. Since the semiconductor device is required to operate both reliably and stably for a long period of time, the contamination of the semiconductor substrate surface, which is believed to be the major reason for the deterioration of reliability, must be prevented by covering the surface of the semiconductor substrate with silicon dioxide film. In this regard, silicon dioxide is the insulative material which is most frequently used for such covering, namely for surface passivation.
It is also known that a single film of silicon dioxide cannot effectively prevent the surface of the semiconductor substrate from being contaminated, which is caused by contaminants, such as moisture, sodium ions, etc., from outside of the semiconductor device. Accordingly, an upper film of alumina (Al.sub.2 O.sub.3) is overlapped on a lower film of silicon dioxide, so as to cover with the upper film the silicon dioxide film which is not highly resistant against the contamination mentioned above. Instead of the alumina film, a phosphorus glass layer, a boron glass layer or a lead glass layer may be formed on the silicon dioxide film. The formation of the phosphorus glass layer or phosphorus treatment can be simply carried out and effectively passivates the semiconductor surface. Although the phosphorus treatment is most frequently employed in the industrial production of semiconductor devices, the surface of the passivation film, on which the phosphorus is impregnated, is disadvantageously hygroscopic and, hence, unstable.
It is also known that the silicon dioxide film formed by directly oxidizing the silicon substrate is superior to an insulative film directly deposited on the silicon substrate by the CVD process. This is because, the CVD process involves a contamination of the interface between the film and substrate due to vapor sources.
C. Gate Insulation Film of MISFET
In the MIS (metal-insulator-semiconductor) type FETs (field effect transistors), the gate insulation films conventionally used include a silicon dioxide film, a double structure film composed of a lower silicon dioxide layer and an upper silicon nitride or oxynitride layer, and a silicon oxide film, which is subjected to the phosphorus treatment mentioned in the item entitled Surface Passivation, above. The silicon dioxide film used for the gate insulation film of MISFETs is formed by thermal oxidation of silicon. However, since silicon dioxide is not inherently dense, MISFETs with excellent properties and large scale integrated circuits cannot be produced from the silicon dioxide film. In the double structure film mentioned above, structural defects are liable to form at the interface of the two layers, with the result that the structural defects act as capturing centers of carriers. Due to the structural defects, the operation of the MISFETs having a gate insulation film of the double structure becomes unstable. When a high electric potential amounting to 10.sup.6 V/cm is applied to the gate insulation film of the MISFETs, the instability problem due to the structural defects is serious, because the threshold voltages (V.sub.TH) of the gate are scattered and vary during the operation of the MISFETs.
When the conventional silicon dioxide film, which is sub]ected to the phosphorus treatment, is sub]ected to a high electric potential, polarization is induced in the silicon dioxide film, with the result that the electric potential of the semiconductor substrate surface is varied. This variation of the electric potential results in instability of the MISFETs.
D. Mask against Diffusion
Diffusion of impurities into a semiconductor substrate is an indispensable step for producing a semiconductor device. Diffusion is frequently carried out by a selective diffusion technique in which an insulative mask is used.
It is known that a thermal oxidation film of silicon dioxide (SiO.sub.2) can be used for the diffusion mask, but that such film cannot exhibit masking effects against particular kinds of impurities, for example, gallium or boron. With regard to gallium, the gallium atoms enter the silicon dioxide film and then the underlying semiconductor substrate. The impurity boron, which is diffused under a hydrogen atmosphere into the semiconductor substrate, also enters the silicon dioxide film through the auxiliary action of the hydrogen. The thermal oxidation film of silicon dioxide cannot, therefore, achieve the selective diffusion of gallium and boron.
It is also known that the film of silicon nitride (Si.sub.3 N.sub.4) formed by the CVD process can be used for the diffusion mask. However, a thin silicon nitride film which is formed by the CVD process on the silicon substrate lacks uniformity, while a thick silicon nitride film is liable to generate thermal stress or cracks between the semiconductor substrate and the silicon nitride film.
E. Conductor deposited on Insulative Film
In the production of semiconductor devices, metallic material is frequently deposited on an insulative film for the purposes of, for example, bonding metallic leads on a semiconductor substrate provided with an insulative film and producing the multi-layer wiring structure of integrated circuits. Since silicon dioxide (SiO.sub.2) is frequently used as the insulative film metals having superior adhering property with the silicon dioxide, such as aluminum (Al), molybdenum (Mo), polycrystalline silicon, chromium (Cr) and titanium (Ti), are used for the conductor material. The more the metals are oxidation reactive, the higher the bonding strength between the insulative film and the metal. At the interface between the highly oxidation-reactive metal film and the silicon dioxide film a transitional layer is created after the deposition of the metal film in which layer the metal is oxidized by the oxygen of the silicon oxide. It is believed by experts in the field of semiconductor technology that the adhering property of the metals mentioned above is increased by the transitional layer. However, insulative properties of the thin silicon dioxide film are disadvantageously deteriorated due to the reaction between the metal and silicon dioxide, namely, by the removal of oxygen from the silicon dioxide film.
F. Capacitor of Semiconductor Device
The conventional capacitor for the semiconductor devices had a structure in which an insulative film was deposited on the metal or semiconductor substrate and was coated by a counter electrode made of metal, etc. Since the capacitor for the semiconductor devices is required to be compact and inexpensive, and to have a large capacitance, a film made of silicon dioxide (SiO.sub.2), alumina (Al.sub.2 O.sub.3) and silicon nitride (Si.sub.3 N.sub.4) was conventionally used for the insulative film. This is because silicon dioxide can be produced simply by heating the semiconductor silicon substrate in an oxidizing atmosphere, and the so produced silicon dioxide film can be thin and is free from such defects as pinholes. However, the thermally oxidized film of silicon is disadvantageous because of the fact that such film has a low dielectric constant, is not highly resistant to emission of radiation and is not highly resistant to contamination from outside the semiconductor device.
The alumina (Al.sub.2 O.sub.3) of the above mentioned film is generally formed by the CVD process or an anodic oxidation process and exhibits a high dielectric constant. However, when the alumina film is thin, a leak current through the film is too large to allow control of the loss of current to an acceptable level.
The silicon nitride (Si.sub.3 N.sub.4) of the above mentioned film can be formed by a thermal nitridation of silicon, but the thickness of such nitride film is limited, so that the maximum allowable voltage of such nitride film raises a problem.
It is also known that films of silicon mono-oxide (SiO), tantalum oxide (Ta.sub.2 O.sub.5) and titanium dioxide (TiO.sub.2) can be used for the capacitor. However, the qualities of the films of these oxides are poor.